1. Field of the Invention
The present invention consists in a fiber optic transmission system and a transmission method for a system of this kind.
2. Description of the Prior Art
The invention concerns fiber optic transmission systems. In the remainder of the present description, "long distance" or "great distance" means distances exceeding 6,000 km, which are typical of the lengths used for transoceanic transmission systems. "High bit rate" means bit rates greater than 5 Gbit/s, which are the bit rates currently envisaged for long distance transmission systems. Specific dispersion problems arise in high bit rate long distance fiber optic transmission systems.
It has been proposed, as for transmission systems, to use for the transmission of NRZ (no return to zeros or RZ (return to zero) signals a wavelength corresponding to normal dispersion conditions of the optical fiber, i.e. a negative dispersion coefficient. For high bit rates and long distances, this type of transmission is not feasible with present day optical fibers: the penalties due to dispersion over distances exceeding a few thousand kilometers are too great. It is possible to provide repeaters at short intervals, for example intervals in the order of 65 km for a total transmission length of 6,000 km and less than 45 km for a total transmission length of 8,000 km, to regenerate the signals. This solution is difficult to use for transoceanic links in which the repeaters are impossible to replace or to repair. Nor is it economically viable for long distances.
Another solution is to use soliton format signals or pulses. The transmission of soliton pulses or solitons in the abnormal dispersion part of an optical fiber is known in itself. Solitons are pulse signals having a sech" shape. With this pulse shape, the non-linearity in the corresponding part of the fiber compensates the dispersion of the optical signal. The transmission of solitons is modeled in a manner known in itself using the non-linear Schrodinger equation. The transmission of so-called "black" solitons, consisting of pulse gaps in a continuous signal, in the normal dispersion part of an optical fiber is also known in itself; in this case the solitons have a wavelength such that they propagate with negative chromatic dispersion. Various effects limit the transmission of such pulses, such as the jitter induced by the interaction of the solitons with the noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, Optical Letters, vol. 11 No. 10, pages 665-667. This effect, known as the Gordon-Haus effect, imposes a theoretical limit on the quality or on the bit rate of transmission using solitons. Systems have been proposed using sliding guide filters to control the jitter of the transmitted solitons so that this limit can be exceeded--see for example EP-A-0 576 208, which specifies that only solitons can be guided by sliding filters and that other types of pulse suffer catastrophic energy losses due to the sliding filtering.
The transmission of soliton pulses or solitons in the abnormal dispersion part of an optical fiber is the generally acknowledged solution at present for high bit rate long distance fiber optic transmission systems. Nevertheless, this solution has drawbacks, and in particular implies a spread of the arrival times of the pulses that often represents a heavy penalty. Also, the soliton pulses interact, which limits the usable bit rate by imposing a minimal "distance" between two neighboring solitons--see for example F. M. Mitschke and L. F. Mollenauer, Optical Letters, vol. 22, No. 5, pages 355-357.
As explained above, there is a prejudice in the art as to the feasibility of a high bit rate long distance fiber optic link using the normal dispersion part of the optical fiber.
The invention proposes a novel and unexpected solution to the problem of dispersion in high bit rate long distance transmission systems which avoids the use of soliton pulses. The invention proposes a solution that goes against the general prejudice referred to above and enables the transmission of signals in the normal dispersion part of the fiber.